In recent years, the use of scanners, monitors (color display devices), color printers, color copying machines, and all-in-one units having these is increasing. This type of color image forming apparatuses are often equipped with a three-dimensional color information conversion table (a three-dimensional look Up table, also referred to hereinafter as RGB→CMYK3D-LUT) that converts the image information of signal processing system of the colors red (R), green (G), and blue (B) into an image data of the CMYK signal processing system. This is because, in the image forming apparatus, the configuration is such that the operations are made based on image data of the CMYK signal processing system.
The RGB→CMYK3D-LUT is prepared, for example, from the measured color values (XYZ or Lab) of the original document with n3 patches in which n patches are arranged so that the intensity of each of the three colors RGB increases, and from the scanner signal (RGB) by carrying out matrix processing and interpolation computation processing, and the RGB→CMYK3D-LUT is for converting the RGB signals to XYZ output signals or Lab output signals.
Relating to the image forming apparatus handling the above described color image data, an image processing apparatus has been disclosed in Japanese Unexamined Laid-Open Patent Publication No. H07-236069. According to this image processing apparatus, the color image data inputted from the input device is subjected to color gamut compression processing to suit the color reproduction region of the output device, and the output image data after this color gamut compression is outputted to the output device.
FIG. 32 is a flow chart showing an example of preparing the RGB→CMYK3D-LUT of a conventional example. To begin with, in Step R1 in the flow chart shown in FIG. 32, the input data and the Lab data are read out from the prescribed image memory and expanded in the RAM (Random Access Memory), etc. The input data and the Lab data are expanded in the RAM and the RGB→Lab3D-LUT is prepared.
Next, in Step R2, the CMY data is read in from the image memory and expanded in the RAM. The CMY data is expanded in the RAM and the CMYK→Lab4D-LUT is prepared. Next, the operation moves on to Step R3 in which the CMY→Lab3D-LUT is prepared from the CMYK→Lab4D-LUT based on a GCR (Gray-Component Replacement) algorithm.
Thereafter, the operation moves on to Step R4 in which the Lab value to calculate in the Lab coordinate system is set. At this time, the CMY value corresponding to each of the Lab values of RGB→Lab3D-LUT is computed from the CMY→Lab3D-LUT. Next, the operation moves on to Step R5 in which the Lab values set (inputted) earlier are searched from the CMY→Lab3D-LUT, and a judgment is made as to whether or not the Lab value is within the color gamut of the CMY→Lab3D-LUT (color gamut inside or outside judgment process). When the calculation target point Pin is judged, by this color gamut inside or outside judgment process, to be outside the color gamut of the printer, the operation moves on to Step R6 in which the compression process is executed.
Further, in Step R5 described above, if the calculation target point Pin is judged to be within the color gamut of the printer, the operation moves on to Step R7 in which the CMY values are searched in the CMY→Lab3D-LUT using the Lab value set earlier by inclusion judgment, and the CMY values are calculated corresponding to this inclusion judgment. Thereafter, the operation moves on to Step R8 in which a judgment is made as to whether or not all the computation processing of the CMY values corresponding to the Lab value set earlier has been completed.
If all the computation has not been completed, the operation returns to Step R4 and the processing described above is repeated. If the computation processing of the CMY values for all the Lab values that have been inputted earlier is completed, the RGB→CMY3D-LUT is prepared. Next, the operation moves on to Step R9 in which the RGB→CMY3D-LUT is converted to the RGB→CMYK3D-LUT by the GCR. In this manner it is possible to prepare the RGB→CMYK3D-LUT (this is referred to hereafter as the conventional example 1).
However, the following problems are present in an image processing apparatus according to a conventional example.
i. In an image processing apparatus as in the Japanese Laid-Open Patent Publication or in the conventional example 1, regarding the reproduction of the six fundamental colors of RGBCMY, as a method of eliminating color mixing, there is the method of moving the fundamental color point of the input side to the output side, and carrying out linear interpolation for other parts (hereinafter referred to as the conventional example 2). In FIG. 33(A), the filled circles denote the fundamental color gradations on the input side. The open circles are the gradations on the fundamental color output side. In actuality, the fundamental color gradation is not as indicated by a straight line in FIG. 33(A) but is non-linear as indicated in FIG. 33(B).
Because of this movement and linear interpolation, regarding the reproduction of fundamental colors, although in appearance it is thought that color mixing is eliminated, the actual color gradations of printers and scanners are usually not linear in the Lab space, but are bent as shown in FIG. 33(B). As a consequence, the method of moving the fundamental colors and carrying out linear interpolation for other parts results in color mixing with the intermediate gradation parts of fundamental colors not matching as is shown in FIG. 34(A) and FIG. 34(B). In the conventional example 2, the filled circles denote the fundamental color gradations on the input side. The open circles are the gradations on the fundamental color output side. In actuality, the fundamental color gradation is not as indicated by a straight line in FIG. 34(A) but is non-linear as indicated in FIG. 34(B). In other words, even the maximum gradation of the output side fundamental color is made to match with the maximum gradation of the input side fundamental color, there will be a difference in the color gamuts of the input side fundamental colors and the color gamuts of the output side fundamental colors. In FIG. 34(B), the horizontal axis corresponds to the color saturation a* of magenta (M) color and the vertical axis corresponds to the color saturation b* of yellow (Y) color. Therefore, although the fundamental color points match, other gradations do not necessarily match. This becomes the cause of color mixing.
ii. When color mixing of other colors occurs in the reproduction of fundamental colors, not only the image is bad to see, but also can cause wasteful consumption of the toner. For example, when reproducing the fundamental color R, instead of being reproduced using only colors Y and M, if the color C is also added, the consumption of the toner of that color C will be wasteful.